Steve,
A 2016 presentation named
An Approach to the Patient Refractory to Platelets Transfusion may have some answers to your question.
It talks about using HLA-matched platelet donors (based on HLA A and HLA B), about using donors in the same "cross-reactive groups" (CREGs), and about finding HLA-antigen negative platelets.
The presentation was by a Dr. Harold Alvarez, and I suspect that
this is him.